1. Field of the Invention
This invention relates to an improvement of bead structure of a pneumatic radial tire having carcass plies made of steel cords for heavy duty vehicles such as trucks, light trucks, buses and so forth and more particularly to a fundamental improvement of the contact part between a bead region, whose deformation is suppressed by a rim flange, and a lower side region, which undergoes easy load deformation, to eliminate the occurence of separation failure.
2. Prior Art
In a conventional pneumatic radial tire for heavy duty vehicles, carcass plies of a tire are turned up around a bead core and terminate at a lower region of a side wall and turn-ups of the carcass plies are generally covered by a reinforcing strip of rubberized steel cords extending to a side wall. In such a structure, a great difference or dislocation in rigidity occurs between both at the ends of the turn-up of the carcass plies or at the upper end of the reinforcing strip of rubberized steel cords and a surrounding rubber. The dislocation in rigidity generates a shearing stress due to a tire deformation caused by repeated load stress resulting from the rotation of tires. Consequently, separation occurs between the steel cords and a surrounding rubber due to the shearing stress.
Some proposals have been made to overcome the problems. In the Japanese Patent Applications laid open under No. 53-119501 and No. 55-106806, high hardness rubber is disposed adjacent to both ends of carcass ply turn-ups and of a reinforcing strip of metallic cords as to cover both ends so that the dislocation in rigidity occurs between the metallic cords and the surrounding rubber can be decreased so as to decrease the concentration of strain at the upper ends of the metallic cords and to prevent a separation.
According to the Japanese Utility Model Publication No. 52-48482, the upper end of a reinforcing layer of steel cords is covered with textile cord fabric so as to prevent the free upper end of the steel cords from moving and thereby to prevent them from being in contact with a surrounding rubber and thereby intended to suppress the inducement of separation and to eliminate a separation problem.
The above described proposals for preventing a separation are directed to obtain a separation resistance structure by reinforcing the places liable to cause a separation. But they have not settled the problem substantially because the concentration points of stress are merely transferred to some other points so that a separation is induced in a different manner.